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Hack the Scosche FMT4 FM Transmitter! 


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Hack the Scosche FMT4 FM 


Written By: Chris 


Clamp (Squeeze clamp, c-clamp, duct 

Jeweler's screwdriver set (1) 

• Multimeter (1) 

Soldering/desoldering tools (1) 


Various colors of small-gauge wire (I 
used 22g cat 5 wire) (1) 

A compact 12v -> 3v regulator (1) 
/ built a variable-output power supply out 
of Radio Shack parts. If you want my 
power supply, pick up a Radio Shack 
LM317T adjustable regulator, about 230 
ohms' worth of fixed resistor (I used 2 
470-Q Radio Shack 271 1 1 15's in 
parallel), a 0.1 f cap, a 1 f cap, a 5k- 
Q pot and some disused 22 gauge cat 5 

A cigarette lighter adapter (1) 
...or whatever you need to connect to 
your 12v power source. Radio Shack 
sells clamp-style cigarette lighter 
adapters, or you may have one your can 
cut up and repurpose. 

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Hack the Scosche FMT4 FM Transmitter! 


I suggest that anyone who is interested in this project do the V2.0 version instead. It is 
easier, safer for the PCB, and should yield practically the same results. The SWR is 
probably higher, but I can't tell a difference in the field. 

Do you want a cheap car FM transmitter with (unlike most of the products you can buy) 
great audio quality and great signal strength? You want to get yourself a Scosche FMT4 and 
hack it! 

It is worth pointing out that someone who is willing to do this level of work could more easily 
change their car stereo for one with an AUX input, but my way is cooler and cheaper. :p 

I am not someone who normally hacks stuff (as you'll be able to tell when you see the 
soldering photos :P ), but I was annoyed by the lack of a decent cheap car FM transmitter. 
Annoyance is the mother of invention, apparently. :p 

This is technically illegal in the US . but so is transporting dentures , so take it with a grain of 
salt, I guess. 

My suggestion for avoiding legal issues would be this: "Don't be a jerk". Use an unoccupied 
frequency, and use the minimum power that you need to be happy. This can mean 
shortening the antenna (use a quarter-wave or an eighth-wave instead of a dipole), or 
(possibly; unconfirmed) lowering the input power. 

A brief test led me to believe that transmit power increases as input power increases, but 
I've never seen a schematic of this thing (or a datasheet for the chip). At some point I will try 
to make time to check reception at various points with various input voltages to validate this 
theory. (Did you ever wish you had an oscilloscope?) 

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Hack the Scosche FMT4 FM Transmitter! 

Step 1 — Hack the Scosche FMT4 FM Transmitter! 

• Update: I came up with a much 
easier method that requires far less 
dangerous (to the PCB) soldering 
and still works great. Unfortunately, 
I didn't photograph the process, but 
I'll take some "after" pictures and 
post them up under a separate 

Step 2 

• 1) Pop front cover (pry it up carefully with a couple of tiny screwdrivers or a knife and a 
screwdriver; it's just glued down). 

• 2) Scrape adhesive to expose four screws. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 3 

• Remove screws. 

Step 4 

• Remove cover. It just pops out. 
There is a little integral clip 
securing the rubber strain relief 
device at the top; it pulls straight 
off also. 

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Hack the Scosche FMT4 FM Transmitter! 


Step 5 

• CAUTION. The next step is pretty much certain to result in the destruction of your 
FMT4 if you are not familiar with basic soldering. Google it. You are about to 
desolder a large mechanical joint in close proximity to several small surface joints. If you 
have the iron on the surface for more than a second, you are doing it wrong. 

• My new FMT4 hack doesn't require this desolder. I'll post it up when my camera battery 
is done charging. :P 

Step 6 

• The battery contacts on the "top" 
(audio cable) side of the FMT4 are 
one solid piece, with two large 
soldered connections through the 
PCB. It needs to go. Be gentle and 
avoid breaking the tiny barely- 
insulated audio lines, which are a 
PITA to fix. Start by gently popping 
the "bottom" of the board (don't rip 
the switch out of the side; see 

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Hack the Scosche FMT4 FM Transmitter! 

Step 7 

• Desolder the battery connector part 
one: Remove excess solder. Using 
well-preheated desoldering braid or 
a vaccuum solder sucker, get the 
excess solder off of these two 
joints quickly. You have some 
delicate stuff in close proximity. Do 
it fast or bail and reconsider your 

Step 8 

• Pull the PCB off of the battery connector terminals. This is the hardest part. There is a 
significant solder blob underneath the PCB, but you need to be sparing with the heat. You 
also need to use one hand to apply pressure to the PCB. Clamp the case to your work 
surface and do it in stages (left, right). It took me 5 iterations. 

• I did not have the requisite number of hands and/or tripods to take a photo of several 
steps; sorry. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 9 

• Pop the PCB out of the case. Use 
leverage from near the switch so 
that you don't damage the switch. 

• Success! The hard part is over. 

Step 10 

• Remove the battery cover. Go nuts 
with the heat; just don't melt the 
screw posts. Desolder the battery 
posts that were in the PCB and 
yank them out through the other 
side of the case. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 11 

• Optional: If you're upgrading the 
antenna, desolder the stock 
antenna wire. CAREFULLY. Don't 
desolder the adjacent audio 
connectors; their little dipped 
insulation can't take much heat. 
This is a surface-blob connection. 
Don't try to desolder it from the 
"top" of the board — do it from the 
side that says "ANT" next to the 

• Clip the old antenna wires short 
so they won't make any 
unexpected contacts. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 12 

• On to the power supply. I don't 
have the components handy to do 
this over. You do not have room to 
put them on a nice tidy PCB. I 
suggest using "liquid electrical 
tape" or some other form of 
paintable rubber for insulation. You 
could just use a 3v regulator 
package, but then it wouldn't be 

• I used a Radio Shack LM317T 
adjustable regulator (which lets 
me choose my input power, but 
makes it a lot harder to fit into 
the battery case). You also need 
about 230 ohms' worth of fixed 
resistor (I used 2 470-Q Radio 
Shack 271 1115's in parallel), a 
0.1 f cap, a 1 f cap, a 5k-Q 
pot, and some disused 22 gauge 
cat 5 wire. 

• Voltage note: I did try this with a 
fixed 3.3v regulator package, 
and while it does work, it's too 
much input voltage and you can 
hear the overdrive. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 13 

• There are 4 connections to make in the power supply: 

• 1) The 0.1 f cap, the wiper on the potentiometer, the 1.0 f cap, ground wire to the 
PCB (make this about 3" long; you'll need to trim it later), and 12V NEGATIVE (aka 
ground). Make this last wire at least as long as you want the power cord to your car to 

• 2) Pin 1 on the LM317T, the 0.1 f capacitor, and 12V POSITIVE in from the car. Make 
this wire at least as long as you want the power cord to your car to be! 

• 3) Pin 2 on the LM317T, the fixed resistor, the 1 .0 f capacitor, and +3.0v out (make 
this wire a few inches long; you'll need to trim it a bit later.) 

• Pin 3 on the LM317T, one of the non-wiper pins on the varistor, and the fixed resistor. 

• Make these four connections temporarily (good mechanical connections with no solder and 
no tape). Check it VERY CAREFULLY for shorts before going on to the next step. Even a 
slight jiggle could create a short if your mechanical connections aren't solid. You can use a 
breadboard for testing if you want something that you can plug in more safely. 

• Bear in mind that when you actually assemble it, it needs to fit into the FMT4 battery 
compartment, so there won't be any fancy PCB or breadboard. ;) 

Step 14 

• Connect the assembled power supply to a +10-20v power source (such as a car cigarette 
lighter), and put a multimeter on the output wires. Adjust the pot for +2.4V. Once it works 
well, take the input power away, work out how to mush the whole mess into the battery 
compartment, and solder all the joints. Apply liquid tape to prevent shorts. 

• Before soldering, ensure that you can get to your pot screw without creating any shorts. 
You may wish to adjust this after you have it all together. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 15 

• If you are replacing the antenna, cut a new antenna wire to the desired length, thread it 
through the battery cover with the power wires, and surface-mount it where the old antenna 
came off. 

• Cheat sheet: 88.1 MHz fullwave: 128" Dipole: 64" 107.9 fullwave: 104" Dipole: 52" 

Step 16 

• Slide the 12v positive, ground, and 
antenna wires through the slot in 
the battery cover, and slide the 
battery cover onto the wires. 

Step 17 

• Dip "+" in flux and ensure that he has a good mechanical connection. Solder that puppy on. 
Now do the same for "-". 

Step 18 

• Poke the antenna wire through the same hole you poked "+" through. Very carefully solder 
it on. Woe be unto you if you desolder the audio connectors like I did. They are no fun to 
put back on correctly. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 19 

• Stick the battery slider switch 
cover on the switch and jam the 
whole thing back into the case. 
Line up the screw holes. Check 
your solders. I'm sure that your 
superior soldering skills resulted in 
no brittle cold solder joints, but 
check it anyway. ;) 

Step 20 

• Check for shorts. Use your eyeballs. If in doubt, apply liquid electrical tape. Do not apply 
power first. :p 

Step 21 

• Put the appropriate end on the 12v wires. I used a cigarette lighter adapter. 

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Hack the Scosche FMT4 FM Transmitter! 

Step 22 

• Reassemble the case. 

• Check that your power and 
antenna wires have proper strain 
relief. I actually just put a big 
knotty twist in each one on the 
inside of the case. 

• Push the audio wires into the 
chassis with one hand, and put 
the plastic cover back on with 
the other. Check that the tiny 
audio wires are not going to get 
caught on a screw post. The 
cover snaps onto the rubber 
strain relief connector on the 
audio/antenna cable. 

• Put the four screws back in. I 
was going to say that a 
magnetized screwdriver will be 
your friend here, but I am 
guessing that anyone who has 
read this far has already 
magnetized most of their 
jewelers' screwdrivers. 

• Stick the front back on. 
Hopefully your frame will have 
less soldering iron burns than 
mine did. :p 

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Hack the Scosche FMT4 FM Transmitter! 

Step 23 

• Plug it in and test it. You should get crystal clear FM reception. If it's a little shy, try 
replacing the antenna (see above; that's what the brown wire is in the photo), or increasing 
the input voltage by turning the screw on the pot. I've gone up to +3.0v with no apparent ill 
effects. +3.3 is too much. 

This document was last generated on 2012-10-31 10:39:38 AM. 

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